Some writing merely communicates information--a sign on the street indicates where drivers should STOP. Composition, on the other hand, involves writing with more sophisticated effects. To quote Katie Wood Ray, composition makes use of "genre, form, sound, and the conventions of the language system" to achieve an effect more layered and significant than anything tax forms and street addresses could ever accomplish.
Writing is composition if it involves a purpose that requires these kinds of sophisticated techniques.
Writing is composition if it involves a purpose that requires these kinds of sophisticated techniques.